Because Charlie was convinced he had to die in order for everything else to fall into place. He viewed it as his fate, and he had already resigned himself to it (as he told the two Others earlier in the episode).

Plus, that hatch looked like it could only be sealed from the inside, anyway -- otherwise Desmond could've just walked up to it while the two Others were in there arguing and locked 'em in.

WEll that was predictable.
I started hating these guys when they sent a post out on myspace telling everyone to buy their album and if already had bought it then buy another copy cause the album is just that good. It pissed me off and I almost threw away my copy. But I don't like throwing away CDs. But I won't be buying the new one they can suck it!